The first film made by an Afghan director in the wake of the collapse of Taliban power has won the influential Sutherland trophy at the London film festival.

Osama, the debut feature from Siddiq Barma, picked up the BFI award during the closing gala in Leicester Square. The trophy is presented annually to "the most original and imaginative feature film from an international director screened at the festival".

Told through the eyes of a 12-year-old girl who is forced to disguise herself as a boy in order to try and earn a living, Osama was rewarded by the judges for revealing the harshness of life under the Taliban "in a style which is direct, accessible and unflinching".

The director, Siddiq Barmak, was born in Afghanistan in 1962 and attended film school in Moscow. He then made short films and managed the Afghan Film Organisation (AFO). Forced to flee Afghanistan in 1996, he lived in exile in Pakistan, returning to head the AFO following the fall of the Taliban.

Commenting on the winning title, the festival's artistic director Sandra Hebron said: "Osama is a timely and distinctive piece of film making, bringing a major new talent to the international film making community. The film combines skilful storytelling with striking visuals and strong performances throughout, to powerful and moving effect. I am delighted that it is the winner of the Sutherland trophy."